Ferromagnetic materials are employed in a variety of applications, some of which require a material having a high coercive force and others employing material having low coercive force. In response to these requirements, a variety of ferromagnetic materials have been formulated among which are several forms of ferromagnetic chromium oxide having a tetragonal crystal structure and which have been produced in high coercivity and low coercivity forms. The ferromagnetic species has been identified as chromium dioxide, but the broader term "ferromagnetic chromium oxide" is more suitable for many materials in which substantial modifiers occupy spaces in the crystal lattice. U.S. Pat. No. 2,956,955 to Arthur, Jr., describes a ferromagnetic chromium oxide composition of tetragonal crystal structure which is produced by thermally decomposing chromium trioxide. The material has an intrinsic coercive force range of 35 to 100 Oersteds but by incorporating various modifiers, chromium oxides having a coercivity range of from 26 to 542 Oersteds have been produced; see for example the Ingraham and Swoboda U.S. Pat. Nos. 3,034,988, 3,068,176 and 2,923,683, Oppegard U.S. Pat. No. 2,885,364, Ingraham U.S. Pat. No. 2,923,684, Swoboda U.S. Pat. No. 2,923,685, Balthis, Jr. U.S. Pat. No. 3,449,073, Kobota U.S. Pat. No. 3,243,260, Haines U.S. Pat. No. 3,574,115 and Mihara et al. U.S. Pat. Nos. 3,547,823 and 3,547,824, disclosing the utilization of such modifiers as iron, titanium, vanadium, manganese, cobalt, nickel, fluoride, antimony, ruthenium, tin, alkali metal, alkali metal dichromate, cerium, tellurium, tellurium-tin and tellurium-calcium. Other modifications are obtained by varying the process parameters. For exmaple, Hicks U.S. Pat. No. 3,493,338 describes chromium oxide of zero intrinsic coercivity obtained by low pressure decomposition. Other decomposition processes are disclosed in Hicks U.S. Pat. No. 3,486,851, Hund U.S. Pat. NO. 3,371,043, Arthur, Jr. and Ingraham U.S. Pat. No. 3,117,093 and Cox U.S. Pat. Nos. 3,074,778 and 3,078,147. In addition to the foregoing, various oxidation processes have been disclosed yielding chromium oxide having intrinsic coercivities ranging from 30-445 Oersteds such as described in Cox U.S. Pat. No. 3,278,263 and in Balthis U.S. Pat. No. 3,423,320.
The foregoing developments have been primarily directed toward modifications of the intrinsic coercivity of the chromium oxide, although a few modifiers have been reported to affect the Curie temperature as well. In all cases the modification of the coercivity or Curie temperature were achieved at the expense of a degradation of the magnetic moment. In the case of modifiers of coercivity, moderate degradation of the magnetic moment is tolerable in view of the overall improvement in the magnetic properties. However, no substantial decrease in Curie temperature has been achieved without gross degradation of both the coercivity and the magnetic moment. Recent developments require magnetic materials having high coercivities and magnetic moment but in which the Curie temperature is lowered, or at least retained at its unmodfied level. For example, U.S. Pat. No. 3,364,496 to Greiner, Eichler and Krones describes a Curie point magnetic recording process in which the magnetic recording material is heated to above its Curie point subjected to the magnetic recording filed, and then magnetized by cooling while under the action of the magnetic recording field. When using a polymeric substrate it is very desirable to use ferromagnetic material having a Curie point sufficiently low to avoid thermal decomposition of the substrate.
Unmodified chromium dioxide has a Curie point of about 116.degree. C. A survey of the art reveals that when chromium oxides of substantially lower Curie point are prepared, they generally have low coercivity and magnetic moment in comparison to the high levels desired for magnetic recording. For example, the aforementioned U.S. Pat. No. 3,034,988 discloses that the addition of a combination of vanadium and nickel oxides as modifiers during the preparation of ferromagnetic chromium oxide can produce material having a Curie point of 99.degree. C, but the intrinsic coercivity is only 31 Oersteds and the magnetic moment is low. Similarly, the patent discloses that a modifier combination of vanadium and cobalt oxides results in a decrease of Curie point to 105.degree. C but the material has a coercivity of only 26 Oersteds. In like manner, the addition of manganese (II) sulfate results in a decrease in Curie point to 104.degree. C but a coercivity of only 68 Oersteds. The patent further discloses that the addition of a combination of ferric oxide and chromium phosphate results in a decrease of Curie point to 100.degree. C but a coercivity of only 40 Oersteds and very low magnetic moment. In the aforementioned U.S. Pat. No. 3,068,176, addition of a combination of manganese fluoride and antimony oxide results in a large decrease of Curie point to 87.degree. C, but the material has an intrinsic coercivity of only 57 Oersteds.
The present invention provides ferromagnetic chromium oxide having an intrinsic coercive force above 150 Oersteds and a Curie temperature less than that for unmodified chromium dioxide, i.e., less than 116.degree. C, a novel material not heretobefore available. The oxides also have a sigma range of about 25-110 under VSM conditions (about 15-60 under BH meter conditions), as set forth hereinbelow. The ferromagnetic oxides are obtained by a process in which material comprising a compound of chromium (III) and sulfur in a form as hereinafter defined is heated under conditions to prepare ferromagnetic chromium oxide in the form of fine, acicular particles of tetragonal crystal structure of the rutile type ranging up to 2 microns in length and having a median axial ratio of greater than 2:1. In accordance with present invention, the material comprising chromium (III) is selected from
a. material corresponding to the formula Cr.sub.2 O.sub.x (SO.sub.4).sub.3.sub.-x wherein x ranges from 0 to 2.99, PA1 b. a combination of a compound of chromium (III) combined with oxygen and about 0.1-25 percent by weight, calculated as sulfate ion, of a sulfate compound having the formula R.sub.2 SO.sub.4 wherein R is an organic radical, H or NH.sub.4, PA1 c. a combination of a compound of chromium (III) combined with oxygen and about 0.1-25 percent by weight, calculated as sulfate ion, of a sulfate compound having the formula (M(H.sub.y SO.sub.4).sub.v(y.sub.+1)/2 wherein M is a trivalent or tetravalent cation other than chromium, v is the valence of M and y is O or l, and PA1 d. a combination of a compound of chromium (III) combined with oxygen and about 0.1-25 percent by weight, calculated as sulfate ion, of a non-sulfate material which yields sulfate upon oxidation.